It is known in the art relating to hydraulic lash adjusters that lash adjusters undergo “leak-down” as part of their lash controlling function. “Leak-down” refers to a reduction in oil volume in a high-pressure chamber of the lash adjuster due to the downward stroke of the lash adjuster's plunger in reaction to a valve spring force of an associated cylinder valve. Hydraulic lash adjuster leak-down occurs through a calibrated clearance between a plunger and a guiding bore in the lash adjuster body.
In a multi-cylinder engine, when the engine is stopped, one or more of the cylinder valves are parked on the lift portions, or lobes, of their respective cams. These positions cause the valve springs to be compressed, providing reaction forces that result in lash adjuster leak-down.
After the engine has been shut off for a period of time, the engine oil galleries that feed oil to the hydraulic lash adjusters will drain and draw in air, often leaving an upper portion of the lash adjuster plunger exposed to air. Further, upon cooling of the engine while not running, the pressure in the high-pressure chamber will slowly become lower than atmospheric pressure due to the contraction of oil in the chamber. The slow contraction of the oil in the high-pressure chamber is not enough to open a check valve of the lash adjuster that is disposed between the high-pressure chamber and a reservoir chamber. The slow contraction of the oil, however, is sufficient to draw oil into the high-pressure chamber from the clearance between the plunger and the bore. In some valvetrain designs, there is not a sufficient supply of oil at the top portion of the clearance between the plunger and the bore to prevent air from also entering the clearance.
Upon restarting the engine, the cylinder valves will move from the cam lobes to the base circle, and the associated hydraulic lash adjusters will stroke upward to remove the lash that was produced during the lash adjuster leak-down. The upward stroke of the hydraulic lash adjuster plunger is brought about by the force of a plunger spring. This return stroke of the plunger at restart takes place rapidly and results in a significantly lowered pressure in the high-pressure chamber. Since at this point the oil film between the plunger and the bore is often fragmented or no longer exists, the easiest flow path into the high-pressure chamber is that of air through the clearance. Air in the high-pressure chamber of the lash adjuster then results in inadequate lash adjustment that produces an audible valvetrain noise. This condition remains until the air is purged from the high-pressure chamber, usually in a path back through the plunger to bore clearance.